The plasma deposition of gas barrier layers in plastic bottles includes the deposition of layers, for instance, of amorphous silicon oxide (SiOx) or carbon compounds (e.g. diamond-like carbon “DLC”), with the aim to prevent the permeation of gases, such as CO2 or oxygen, through the plastic material. Additionally, with respect to the inside coating of bottles, e.g. plastic bottles made of polyethylene terephthalate (PET), the migration of foreign substances from the plastic into the product can be prevented. Examples for such foreign substances in connection with PET are, above all, acetaldehyde or antimony. However, also bottle materials based on other synthetic materials contain, for instance, plasticizers which partially only have a weak bond to the substrate material.
In particular after a thermal treatment of the bottles, such as a plasma treatment, the release of undesired substances from the bottles is clearly encouraged. Experience has shown that PET bottles having an inadequate layer quality already have an unpleasant strong smell of acetaldehyde after one thermal treatment or plasma coating process.
In addition, it is the purpose of a plasma coating to prevent the absorption of sterilization media like PAA or H2O2 into the bottle material as this may result in a delayed release of undesired foreign substances into the product contained in the bottle.
The thicknesses of these plasma layers, which will also be referred to as barrier layers below, are within a range of ten to some hundred nanometers and are invisible, in particular in the case of silicon oxide.
The following methods are known for monitoring the coating quality:
Classical measuring devices, e.g. from companies like PreSens or Mocon (Ox-tran), may be used for the detection of permeation substances, e.g. gases, penetrating the bottle wall.
The coating may also be tested with modern layer analyzing methods, e.g. Fourier transform infrared spectroscopy (FTIR) techniques as described in U.S. Pat. No. 6,531,193 or EP 1 273 677, or by ellipsometry (see, for instance, J. Electrochem. Soc., Volume 138, Issue 11, pp. 3266-3275, 1991).
Another method is a so-called acid test in which the bottles are exposed to an aggressive medium, such as concentrated sulfuric acid. Uncoated bottles are attacked by the medium and become dull, while coated bottles show a certain protection against the medium.
All of these methods have the disadvantage that they take a very long time (some minutes to some days), or that the bottle has to be destroyed for analyzing it.
Furthermore, it is known, for instance from US020080292781A and DE102010012501, that a faster inspection of the plasma coating can be accomplished by means of light spectroscopy test methods, where the intensity of specific spectral lines of the plasma contained in the bottle is compared with a predefined reference intensity and, if the spectral line intensities are sufficiently congruent, it will be assumed that the plasma coating has worked. However, this method measures the plasma coating only indirectly, and an accurate, complete plasma coating representing an intact gas barrier cannot be determined with certainty.
Therefore, it is an object of the disclosure to provide an apparatus and a method to improve the inspection of coated containers, for instance of plastic bottles, coated by a plasma treatment.